It is known for the conveying and extraction equipment installed in a longwall, together with the self-advancing support, to be monitored from a control station. During extraction, the pushing grams of the self-advancing support units connected to the conveyor are pressurised so that the cutter exerts continuous pressure on the working face. In order to monitor and control the sequence of operation, the instantaneous position of the cutter in the longwall and the extent to which the pushing rams are extended is measured and transmitted to the control station, which is usually outside the longwall. After the extraction process, the support is moved forward in dependence on the measurements transmitted to the control station.
Coal is usually extracted in stages by cutting successive parts of the longwall. The self-advancing support units are moved forward after the conveyor has advanced by the full length to which the pushing ram is extended. Cutting in stages is advantageous because the exposed roof is carried by the support soon after extraction and because irregularities in the longwall can be compensated by suitable work afterwards. On the other hand, it is difficult to keep the longwall straight. Since extraction and advance operations are carried out in different regions of the longwall, it is difficult to monitor and synchronize the sequences of operations, and this results in idle times for both the cutter and the self-advancing support unit.
On the other hand, cutting in long strokes, covering nearly the entire length of the longwall, results in considerable delays to the self-advancing support, because the support can be advanced only after the entire length of the longwall has been exposed by cutting by an amount equal to the extension length of the pushing rams. In addition, the self-advancing operations interfere with extraction. In the case of long cutter strokes, however, it is easier to keep the longwall straight.
A straight longwall is obtained if the cutter cuts to a substantially uniform depth during each extraction cut. However, the depth of cut cannot be made uniform if the conveyor is continuously pressed by the pushing rams, because the solid coal does not have uniform consistency. The cutter is pressed into soft coal to a greater extent than in parts of a seam containing hard coal. The result is a non-uniform undulating curve which intensifies as the extraction proceeds. Accordingly, DE-AS No. 15 33 776 proposes that the extension length of a pushing ram should be limited to a predetermined cutting depth of the cutter, using a metering cylinder as a quantity-control device. During the extraction travel of the cutter, the supply of pressure medium to the pushing ram is stopped so that the cutter abuts against the column of liquid in the pushing ram. After extraction, the pushing ram is supplied via the metering cylinder with a metered amount of pressure fluid corresponding to extension as far as the preset cutting depth. However, there is no coupling between the extraction process and the self-advancing work.
The object of the invention therefore is to devise an efficient method of obtaining a controlled sequence, well matched in time and space, of extraction and self-advancing support work in an automated longwall operation.